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DavidS
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GPIO Power Safety Margin?

Wed Mar 15, 2017 4:26 pm

I realize that it is not in any of the documents, at least not that I can find:

For running external items off of the GPIO's 5V power what is the maximum safe draw (10A power supply) for the various models of the Raspberry Pi?

I would assume that at least on the 3B and 2B it would be safe to draw 1A at 5V, though I would like to have some verification. And I have no clue for the original model A/B or the A+/B+ or Zero.

For a few projects to reduce the amount of power source wiring I would like to power some things off of the RPi, mostly 28BYJ-48 5v steppers, small servos, and Parallax Propeller P8X32As. Though rather than smoke testing an RPi for this I would like some numbers. Note that for the steppers and servos I am using ULN2803's for the drivers.

Thank you very much.
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Burngate
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 6:42 pm

About the only thing we're sure of is that the 3B has a 2.5A fuse (hold current 2.5A, trip current 5A)
The B+ & 2B has a 2A fuse (4A trip)
Original B fuse says 1A1
Zero has no fuse.

Out of those currents have to taken whatever the SoC uses, and USB devices have their share, so we're left with some undefined current left.

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DavidS
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 8:06 pm

Burngate wrote:About the only thing we're sure of is that the 3B has a 2.5A fuse (hold current 2.5A, trip current 5A)
The B+ & 2B has a 2A fuse (4A trip)
Original B fuse says 1A1
Zero has no fuse.

Out of those currents have to taken whatever the SoC uses, and USB devices have their share, so we're left with some undefined current left.
Ok that is actually helpfull. So now I just need more precise numbers on how much is taken by the on board components (SoC, SDRAM, USB-Hub+Ethernet, etc), then it is just a matter of double checking the current draw of the USB devices used.

So to rephrase, being within good safety margins, how much current is available on the 5 volt side (USB AND GPIO)after subtracting what the RPi B/A+/B+/2B/3B/Zero on board components use, worse case draw for the on board components?
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dgordon42
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 9:29 pm

This may help you figure out what your Pi will consume, and therefor how much you have left to play with before reaching the fuse limitation mentioned by @Burngate.
If you read down through it, it gives boot, idle, and stress current draws for most models.

You also need to be a little careful about what you power from the Pi. Will your device draw a steady current, or can it demand a surge from time to time? Has your device got any inductive properties that could send a back EMF into your Pi, things like relays or motors.

Good Luck,
Dave.

mattmiller
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 9:40 pm

You can split your 5V feeding your Pi and take it direct to the servos and steppers thereby eliminating those from the equation - I normal only power 2 servos/steppers via the Pi - after that - I move to supplying them direct instead of via the Pi.

gregeric
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 9:42 pm

Better to split the clean logic supply from the dirty motor supply rails, not try to use one rail for both.

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DavidS
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 11:24 pm

dgordon42 wrote:This may help you figure out what your Pi will consume, and therefor how much you have left to play with before reaching the fuse limitation mentioned by @Burngate.
If you read down through it, it gives boot, idle, and stress current draws for most models.

You also need to be a little careful about what you power from the Pi. Will your device draw a steady current, or can it demand a surge from time to time? Has your device got any inductive properties that could send a back EMF into your Pi, things like relays or motors.

Good Luck,
Dave.
And the reason you always isolate a motor from the RPi by a driver, and a few discretes. Thank you for the link.
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DavidS
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Re: GPIO Power Safety Margin?

Wed Mar 15, 2017 11:27 pm

gregeric wrote:Better to split the clean logic supply from the dirty motor supply rails, not try to use one rail for both.
Yes I had thought about that as well. It is one of those issues that there is debate on both directions.
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